content: // Protocol Buffers - Google's data interchange format
  // Copyright 2008 Google Inc.  All rights reserved.
  // https://developers.google.com/protocol-buffers/
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  // notice, this list of conditions and the following disclaimer.
  //     * Redistributions in binary form must reproduce the above
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  // in the documentation and/or other materials provided with the
  // distribution.
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  // contributors may be used to endorse or promote products derived from
  // this software without specific prior written permission.
  //
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  package com.google.protobuf;
  
  import static com.google.protobuf.UnsafeUtil.addressOffset;
  import static com.google.protobuf.UnsafeUtil.hasUnsafeArrayOperations;
  import static com.google.protobuf.UnsafeUtil.hasUnsafeByteBufferOperations;
  import static java.lang.Character.MAX_SURROGATE;
  import static java.lang.Character.MIN_HIGH_SURROGATE;
  import static java.lang.Character.MIN_LOW_SURROGATE;
  import static java.lang.Character.MIN_SUPPLEMENTARY_CODE_POINT;
  import static java.lang.Character.MIN_SURROGATE;
  import static java.lang.Character.isSurrogatePair;
  import static java.lang.Character.toCodePoint;
  
  import java.nio.ByteBuffer;
  import java.util.Arrays;
  
  /**
   * A set of low-level, high-performance static utility methods related
   * to the UTF-8 character encoding.  This class has no dependencies
   * outside of the core JDK libraries.
   *
   * <p>There are several variants of UTF-8.  The one implemented by
   * this class is the restricted definition of UTF-8 introduced in
   * Unicode 3.1, which mandates the rejection of "overlong" byte
   * sequences as well as rejection of 3-byte surrogate codepoint byte
   * sequences.  Note that the UTF-8 decoder included in Oracle's JDK
   * has been modified to also reject "overlong" byte sequences, but (as
   * of 2011) still accepts 3-byte surrogate codepoint byte sequences.
   *
   * <p>The byte sequences considered valid by this class are exactly
   * those that can be roundtrip converted to Strings and back to bytes
   * using the UTF-8 charset, without loss: <pre> {@code
   * Arrays.equals(bytes, new String(bytes, Internal.UTF_8).getBytes(Internal.UTF_8))
   * }</pre>
   *
   * <p>See the Unicode Standard,</br>
   * Table 3-6. <em>UTF-8 Bit Distribution</em>,</br>
   * Table 3-7. <em>Well Formed UTF-8 Byte Sequences</em>.
   *
   * <p>This class supports decoding of partial byte sequences, so that the
   * bytes in a complete UTF-8 byte sequences can be stored in multiple
   * segments.  Methods typically return {@link #MALFORMED} if the partial
   * byte sequence is definitely not well-formed, {@link #COMPLETE} if it is
   * well-formed in the absence of additional input, or if the byte sequence
   * apparently terminated in the middle of a character, an opaque integer
   * "state" value containing enough information to decode the character when
   * passed to a subsequent invocation of a partial decoding method.
   *
   * @author martinrb@google.com (Martin Buchholz)
   */
  // TODO(nathanmittler): Copy changes in this class back to Guava
  final class Utf8 {
  
    /**
     * UTF-8 is a runtime hot spot so we attempt to provide heavily optimized implementations
     * depending on what is available on the platform. The processor is the platform-optimized
     * delegate for which all methods are delegated directly to.
     */
    private static final Processor processor =
        UnsafeProcessor.isAvailable() ? new UnsafeProcessor() : new SafeProcessor();
  
    /**
     * A mask used when performing unsafe reads to determine if a long value contains any non-ASCII
     * characters (i.e. any byte >= 0x80).
     */
    private static final long ASCII_MASK_LONG = 0x8080808080808080L;
  
    /**
     * Maximum number of bytes per Java UTF-16 char in UTF-8.
     * @see java.nio.charset.CharsetEncoder#maxBytesPerChar()
     */
    static final int MAX_BYTES_PER_CHAR = 3;
  
    /**
     * State value indicating that the byte sequence is well-formed and
     * complete (no further bytes are needed to complete a character).
     */
    public static final int COMPLETE = 0;
  
    /**
     * State value indicating that the byte sequence is definitely not
     * well-formed.
     */
    public static final int MALFORMED = -1;
  
    /**
     * Used by {@code Unsafe} UTF-8 string validation logic to determine the minimum string length
     * above which to employ an optimized algorithm for counting ASCII characters. The reason for this
     * threshold is that for small strings, the optimization may not be beneficial or may even
     * negatively impact performance since it requires additional logic to avoid unaligned reads
     * (when calling {@code Unsafe.getLong}). This threshold guarantees that even if the initial
     * offset is unaligned, we're guaranteed to make at least one call to {@code Unsafe.getLong()}
     * which provides a performance improvement that entirely subsumes the cost of the additional
     * logic.
     */
    private static final int UNSAFE_COUNT_ASCII_THRESHOLD = 16;
  
    // Other state values include the partial bytes of the incomplete
    // character to be decoded in the simplest way: we pack the bytes
    // into the state int in little-endian order.  For example:
    //
    // int state = byte1 ^ (byte2 << 8) ^ (byte3 << 16);
    //
    // Such a state is unpacked thus (note the ~ operation for byte2 to
    // undo byte1's sign-extension bits):
    //
    // int byte1 = (byte) state;
    // int byte2 = (byte) ~(state >> 8);
    // int byte3 = (byte) (state >> 16);
    //
    // We cannot store a zero byte in the state because it would be
    // indistinguishable from the absence of a byte.  But we don't need
    // to, because partial bytes must always be negative.  When building
    // a state, we ensure that byte1 is negative and subsequent bytes
    // are valid trailing bytes.
  
    /**
     * Returns {@code true} if the given byte array is a well-formed
     * UTF-8 byte sequence.
     *
     * <p>This is a convenience method, equivalent to a call to {@code
     * isValidUtf8(bytes, 0, bytes.length)}.
     */
    public static boolean isValidUtf8(byte[] bytes) {
      return processor.isValidUtf8(bytes, 0, bytes.length);
    }
  
    /**
     * Returns {@code true} if the given byte array slice is a
     * well-formed UTF-8 byte sequence.  The range of bytes to be
     * checked extends from index {@code index}, inclusive, to {@code
     * limit}, exclusive.
     *
     * <p>This is a convenience method, equivalent to {@code
     * partialIsValidUtf8(bytes, index, limit) == Utf8.COMPLETE}.
     */
    public static boolean isValidUtf8(byte[] bytes, int index, int limit) {
      return processor.isValidUtf8(bytes, index, limit);
    }
  
    /**
     * Tells whether the given byte array slice is a well-formed,
     * malformed, or incomplete UTF-8 byte sequence.  The range of bytes
     * to be checked extends from index {@code index}, inclusive, to
     * {@code limit}, exclusive.
     *
     * @param state either {@link Utf8#COMPLETE} (if this is the initial decoding
     * operation) or the value returned from a call to a partial decoding method
     * for the previous bytes
     *
     * @return {@link #MALFORMED} if the partial byte sequence is
     * definitely not well-formed, {@link #COMPLETE} if it is well-formed
     * (no additional input needed), or if the byte sequence is
     * "incomplete", i.e. apparently terminated in the middle of a character,
     * an opaque integer "state" value containing enough information to
     * decode the character when passed to a subsequent invocation of a
     * partial decoding method.
     */
    public static int partialIsValidUtf8(int state, byte[] bytes, int index, int limit) {
      return processor.partialIsValidUtf8(state, bytes, index, limit);
    }
  
    private static int incompleteStateFor(int byte1) {
      return (byte1 > (byte) 0xF4) ?
          MALFORMED : byte1;
    }
  
    private static int incompleteStateFor(int byte1, int byte2) {
      return (byte1 > (byte) 0xF4 ||
              byte2 > (byte) 0xBF) ?
          MALFORMED : byte1 ^ (byte2 << 8);
    }
  
    private static int incompleteStateFor(int byte1, int byte2, int byte3) {
      return (byte1 > (byte) 0xF4 ||
              byte2 > (byte) 0xBF ||
              byte3 > (byte) 0xBF) ?
          MALFORMED : byte1 ^ (byte2 << 8) ^ (byte3 << 16);
    }
  
    private static int incompleteStateFor(byte[] bytes, int index, int limit) {
      int byte1 = bytes[index - 1];
      switch (limit - index) {
        case 0: return incompleteStateFor(byte1);
        case 1: return incompleteStateFor(byte1, bytes[index]);
        case 2: return incompleteStateFor(byte1, bytes[index], bytes[index + 1]);
        default: throw new AssertionError();
      }
    }
  
    private static int incompleteStateFor(
        final ByteBuffer buffer, final int byte1, final int index, final int remaining) {
      switch (remaining) {
        case 0:
          return incompleteStateFor(byte1);
        case 1:
          return incompleteStateFor(byte1, buffer.get(index));
        case 2:
          return incompleteStateFor(byte1, buffer.get(index), buffer.get(index + 1));
        default:
          throw new AssertionError();
      }
    }
  
    // These UTF-8 handling methods are copied from Guava's Utf8 class with a modification to throw
    // a protocol buffer local exception. This exception is then caught in CodedOutputStream so it can
    // fallback to more lenient behavior.
    
    static class UnpairedSurrogateException extends IllegalArgumentException {
      UnpairedSurrogateException(int index, int length) {
        super("Unpaired surrogate at index " + index + " of " + length);
      }
    }
  
    /**
     * Returns the number of bytes in the UTF-8-encoded form of {@code sequence}. For a string,
     * this method is equivalent to {@code string.getBytes(UTF_8).length}, but is more efficient in
     * both time and space.
     *
     * @throws IllegalArgumentException if {@code sequence} contains ill-formed UTF-16 (unpaired
     *     surrogates)
     */
    static int encodedLength(CharSequence sequence) {
      // Warning to maintainers: this implementation is highly optimized.
      int utf16Length = sequence.length();
      int utf8Length = utf16Length;
      int i = 0;
  
      // This loop optimizes for pure ASCII.
      while (i < utf16Length && sequence.charAt(i) < 0x80) {
        i++;
      }
  
      // This loop optimizes for chars less than 0x800.
      for (; i < utf16Length; i++) {
        char c = sequence.charAt(i);
        if (c < 0x800) {
          utf8Length += ((0x7f - c) >>> 31);  // branch free!
        } else {
          utf8Length += encodedLengthGeneral(sequence, i);
          break;
        }
      }
  
      if (utf8Length < utf16Length) {
        // Necessary and sufficient condition for overflow because of maximum 3x expansion
        throw new IllegalArgumentException("UTF-8 length does not fit in int: "
                + (utf8Length + (1L << 32)));
      }
      return utf8Length;
    }
  
    private static int encodedLengthGeneral(CharSequence sequence, int start) {
      int utf16Length = sequence.length();
      int utf8Length = 0;
      for (int i = start; i < utf16Length; i++) {
        char c = sequence.charAt(i);
        if (c < 0x800) {
          utf8Length += (0x7f - c) >>> 31; // branch free!
        } else {
          utf8Length += 2;
          // jdk7+: if (Character.isSurrogate(c)) {
          if (Character.MIN_SURROGATE <= c && c <= Character.MAX_SURROGATE) {
            // Check that we have a well-formed surrogate pair.
            int cp = Character.codePointAt(sequence, i);
            if (cp < MIN_SUPPLEMENTARY_CODE_POINT) {
              throw new UnpairedSurrogateException(i, utf16Length);
            }
            i++;
          }
        }
      }
      return utf8Length;
    }
  
    static int encode(CharSequence in, byte[] out, int offset, int length) {
      return processor.encodeUtf8(in, out, offset, length);
    }
    // End Guava UTF-8 methods.
  
    /**
     * Determines if the given {@link ByteBuffer} is a valid UTF-8 string.
     *
     * <p>Selects an optimal algorithm based on the type of {@link ByteBuffer} (i.e. heap or direct)
     * and the capabilities of the platform.
     *
     * @param buffer the buffer to check.
     * @see Utf8#isValidUtf8(byte[], int, int)
     */
    static boolean isValidUtf8(ByteBuffer buffer) {
      return processor.isValidUtf8(buffer, buffer.position(), buffer.remaining());
    }
  
    /**
     * Determines if the given {@link ByteBuffer} is a partially valid UTF-8 string.
     *
     * <p>Selects an optimal algorithm based on the type of {@link ByteBuffer} (i.e. heap or direct)
     * and the capabilities of the platform.
     *
     * @param buffer the buffer to check.
     * @see Utf8#partialIsValidUtf8(int, byte[], int, int)
     */
    static int partialIsValidUtf8(int state, ByteBuffer buffer, int index, int limit) {
      return processor.partialIsValidUtf8(state, buffer, index, limit);
    }
  
    /**
     * Decodes the given UTF-8 portion of the {@link ByteBuffer} into a {@link String}.
     *
     * @throws InvalidProtocolBufferException if the input is not valid UTF-8.
     */
    static String decodeUtf8(ByteBuffer buffer, int index, int size)
        throws InvalidProtocolBufferException {
      return processor.decodeUtf8(buffer, index, size);
    }
  
    /**
     * Decodes the given UTF-8 encoded byte array slice into a {@link String}.
     *
     * @throws InvalidProtocolBufferException if the input is not valid UTF-8.
     */
    static String decodeUtf8(byte[] bytes, int index, int size)
        throws InvalidProtocolBufferException {
      return processor.decodeUtf8(bytes, index, size);
    }
  
    /**
     * Encodes the given characters to the target {@link ByteBuffer} using UTF-8 encoding.
     *
     * <p>Selects an optimal algorithm based on the type of {@link ByteBuffer} (i.e. heap or direct)
     * and the capabilities of the platform.
     *
     * @param in the source string to be encoded
     * @param out the target buffer to receive the encoded string.
     * @see Utf8#encode(CharSequence, byte[], int, int)
     */
    static void encodeUtf8(CharSequence in, ByteBuffer out) {
      processor.encodeUtf8(in, out);
    }
  
    /**
     * Counts (approximately) the number of consecutive ASCII characters in the given buffer.
     * The byte order of the {@link ByteBuffer} does not matter, so performance can be improved if
     * native byte order is used (i.e. no byte-swapping in {@link ByteBuffer#getLong(int)}).
     *
     * @param buffer the buffer to be scanned for ASCII chars
     * @param index the starting index of the scan
     * @param limit the limit within buffer for the scan
     * @return the number of ASCII characters found. The stopping position will be at or
     * before the first non-ASCII byte.
     */
    private static int estimateConsecutiveAscii(ByteBuffer buffer, int index, int limit) {
      int i = index;
      final int lim = limit - 7;
      // This simple loop stops when we encounter a byte >= 0x80 (i.e. non-ASCII).
      // To speed things up further, we're reading longs instead of bytes so we use a mask to
      // determine if any byte in the current long is non-ASCII.
      for (; i < lim && (buffer.getLong(i) & ASCII_MASK_LONG) == 0; i += 8) {}
      return i - index;
    }
  
    /**
     * A processor of UTF-8 strings, providing methods for checking validity and encoding.
     */
    // TODO(nathanmittler): Add support for Memory/MemoryBlock on Android.
    abstract static class Processor {
      /**
       * Returns {@code true} if the given byte array slice is a
       * well-formed UTF-8 byte sequence.  The range of bytes to be
       * checked extends from index {@code index}, inclusive, to {@code
       * limit}, exclusive.
       *
       * <p>This is a convenience method, equivalent to {@code
       * partialIsValidUtf8(bytes, index, limit) == Utf8.COMPLETE}.
       */
      final boolean isValidUtf8(byte[] bytes, int index, int limit) {
        return partialIsValidUtf8(COMPLETE, bytes, index, limit) == COMPLETE;
      }
  
      /**
       * Tells whether the given byte array slice is a well-formed,
       * malformed, or incomplete UTF-8 byte sequence.  The range of bytes
       * to be checked extends from index {@code index}, inclusive, to
       * {@code limit}, exclusive.
       *
       * @param state either {@link Utf8#COMPLETE} (if this is the initial decoding
       * operation) or the value returned from a call to a partial decoding method
       * for the previous bytes
       *
       * @return {@link #MALFORMED} if the partial byte sequence is
       * definitely not well-formed, {@link #COMPLETE} if it is well-formed
       * (no additional input needed), or if the byte sequence is
       * "incomplete", i.e. apparently terminated in the middle of a character,
       * an opaque integer "state" value containing enough information to
       * decode the character when passed to a subsequent invocation of a
       * partial decoding method.
       */
      abstract int partialIsValidUtf8(int state, byte[] bytes, int index, int limit);
  
      /**
       * Returns {@code true} if the given portion of the {@link ByteBuffer} is a
       * well-formed UTF-8 byte sequence.  The range of bytes to be
       * checked extends from index {@code index}, inclusive, to {@code
       * limit}, exclusive.
       *
       * <p>This is a convenience method, equivalent to {@code
       * partialIsValidUtf8(bytes, index, limit) == Utf8.COMPLETE}.
       */
      final boolean isValidUtf8(ByteBuffer buffer, int index, int limit) {
        return partialIsValidUtf8(COMPLETE, buffer, index, limit) == COMPLETE;
      }
  
      /**
       * Indicates whether or not the given buffer contains a valid UTF-8 string.
       *
       * @param buffer the buffer to check.
       * @return {@code true} if the given buffer contains a valid UTF-8 string.
       */
      final int partialIsValidUtf8(
          final int state, final ByteBuffer buffer, int index, final int limit) {
        if (buffer.hasArray()) {
          final int offset = buffer.arrayOffset();
          return partialIsValidUtf8(state, buffer.array(), offset + index, offset + limit);
        } else if (buffer.isDirect()){
          return partialIsValidUtf8Direct(state, buffer, index, limit);
        }
        return partialIsValidUtf8Default(state, buffer, index, limit);
      }
  
      /**
       * Performs validation for direct {@link ByteBuffer} instances.
       */
      abstract int partialIsValidUtf8Direct(
          final int state, final ByteBuffer buffer, int index, final int limit);
  
      /**
       * Performs validation for {@link ByteBuffer} instances using the {@link ByteBuffer} API rather
       * than potentially faster approaches. This first completes validation for the current
       * character (provided by {@code state}) and then finishes validation for the sequence.
       */
      final int partialIsValidUtf8Default(
          final int state, final ByteBuffer buffer, int index, final int limit) {
        if (state != COMPLETE) {
          // The previous decoding operation was incomplete (or malformed).
          // We look for a well-formed sequence consisting of bytes from
          // the previous decoding operation (stored in state) together
          // with bytes from the array slice.
          //
          // We expect such "straddler characters" to be rare.
  
          if (index >= limit) { // No bytes? No progress.
            return state;
          }
  
          byte byte1 = (byte) state;
          // byte1 is never ASCII.
          if (byte1 < (byte) 0xE0) {
            // two-byte form
  
            // Simultaneously checks for illegal trailing-byte in
            // leading position and overlong 2-byte form.
            if (byte1 < (byte) 0xC2
                // byte2 trailing-byte test
                || buffer.get(index++) > (byte) 0xBF) {
              return MALFORMED;
            }
          } else if (byte1 < (byte) 0xF0) {
            // three-byte form
  
            // Get byte2 from saved state or array
            byte byte2 = (byte) ~(state >> 8);
            if (byte2 == 0) {
              byte2 = buffer.get(index++);
              if (index >= limit) {
                return incompleteStateFor(byte1, byte2);
              }
            }
            if (byte2 > (byte) 0xBF
                // overlong? 5 most significant bits must not all be zero
                || (byte1 == (byte) 0xE0 && byte2 < (byte) 0xA0)
                // illegal surrogate codepoint?
                || (byte1 == (byte) 0xED && byte2 >= (byte) 0xA0)
                // byte3 trailing-byte test
                || buffer.get(index++) > (byte) 0xBF) {
              return MALFORMED;
            }
          } else {
            // four-byte form
  
            // Get byte2 and byte3 from saved state or array
            byte byte2 = (byte) ~(state >> 8);
            byte byte3 = 0;
            if (byte2 == 0) {
              byte2 = buffer.get(index++);
              if (index >= limit) {
                return incompleteStateFor(byte1, byte2);
              }
            } else {
              byte3 = (byte) (state >> 16);
            }
            if (byte3 == 0) {
              byte3 = buffer.get(index++);
              if (index >= limit) {
                return incompleteStateFor(byte1, byte2, byte3);
              }
            }
  
            // If we were called with state == MALFORMED, then byte1 is 0xFF,
            // which never occurs in well-formed UTF-8, and so we will return
            // MALFORMED again below.
  
            if (byte2 > (byte) 0xBF
                // Check that 1 <= plane <= 16.  Tricky optimized form of:
                // if (byte1 > (byte) 0xF4 ||
                //     byte1 == (byte) 0xF0 && byte2 < (byte) 0x90 ||
                //     byte1 == (byte) 0xF4 && byte2 > (byte) 0x8F)
                || (((byte1 << 28) + (byte2 - (byte) 0x90)) >> 30) != 0
                // byte3 trailing-byte test
                || byte3 > (byte) 0xBF
                // byte4 trailing-byte test
                || buffer.get(index++) > (byte) 0xBF) {
              return MALFORMED;
            }
          }
        }
  
        // Finish validation for the sequence.
        return partialIsValidUtf8(buffer, index, limit);
      }
  
      /**
       * Performs validation for {@link ByteBuffer} instances using the {@link ByteBuffer} API rather
       * than potentially faster approaches.
       */
      private static int partialIsValidUtf8(final ByteBuffer buffer, int index, final int limit) {
        index += estimateConsecutiveAscii(buffer, index, limit);
  
        for (;;) {
          // Optimize for interior runs of ASCII bytes.
          // TODO(nathanmittler): Consider checking 8 bytes at a time after some threshold?
          // Maybe after seeing a few in a row that are ASCII, go back to fast mode?
          int byte1;
          do {
            if (index >= limit) {
              return COMPLETE;
            }
          } while ((byte1 = buffer.get(index++)) >= 0);
  
          // If we're here byte1 is not ASCII. Only need to handle 2-4 byte forms.
          if (byte1 < (byte) 0xE0) {
            // Two-byte form (110xxxxx 10xxxxxx)
            if (index >= limit) {
              // Incomplete sequence
              return byte1;
            }
  
            // Simultaneously checks for illegal trailing-byte in
            // leading position and overlong 2-byte form.
            if (byte1 < (byte) 0xC2 || buffer.get(index) > (byte) 0xBF) {
              return MALFORMED;
            }
            index++;
          } else if (byte1 < (byte) 0xF0) {
            // Three-byte form (1110xxxx 10xxxxxx 10xxxxxx)
            if (index >= limit - 1) {
              // Incomplete sequence
              return incompleteStateFor(buffer, byte1, index, limit - index);
            }
  
            final byte byte2 = buffer.get(index++);
            if (byte2 > (byte) 0xBF
                // overlong? 5 most significant bits must not all be zero
                || (byte1 == (byte) 0xE0 && byte2 < (byte) 0xA0)
                // check for illegal surrogate codepoints
                || (byte1 == (byte) 0xED && byte2 >= (byte) 0xA0)
                // byte3 trailing-byte test
                || buffer.get(index) > (byte) 0xBF) {
              return MALFORMED;
            }
            index++;
          } else {
            // Four-byte form (1110xxxx 10xxxxxx 10xxxxxx 10xxxxxx)
            if (index >= limit - 2) {
              // Incomplete sequence
              return incompleteStateFor(buffer, byte1, index, limit - index);
            }
  
            // TODO(nathanmittler): Consider using getInt() to improve performance.
            final int byte2 = buffer.get(index++);
            if (byte2 > (byte) 0xBF
                // Check that 1 <= plane <= 16.  Tricky optimized form of:
                // if (byte1 > (byte) 0xF4 ||
                //     byte1 == (byte) 0xF0 && byte2 < (byte) 0x90 ||
                //     byte1 == (byte) 0xF4 && byte2 > (byte) 0x8F)
                || (((byte1 << 28) + (byte2 - (byte) 0x90)) >> 30) != 0
                // byte3 trailing-byte test
                || buffer.get(index++) > (byte) 0xBF
                // byte4 trailing-byte test
                || buffer.get(index++) > (byte) 0xBF) {
              return MALFORMED;
            }
          }
        }
      }
  
      /**
       * Decodes the given byte array slice into a {@link String}.
       *
       * @throws InvalidProtocolBufferException if the byte array slice is not valid UTF-8.
       */
      abstract String decodeUtf8(byte[] bytes, int index, int size)
          throws InvalidProtocolBufferException;
  
      /**
       * Decodes the given portion of the {@link ByteBuffer} into a {@link String}.
       *
       * @throws InvalidProtocolBufferException if the portion of the buffer is not valid UTF-8.
       */
      final String decodeUtf8(ByteBuffer buffer, int index, int size)
          throws InvalidProtocolBufferException {
        if (buffer.hasArray()) {
          final int offset = buffer.arrayOffset();
          return decodeUtf8(buffer.array(), offset + index, size);
        } else if (buffer.isDirect()) {
          return decodeUtf8Direct(buffer, index, size);
        }
        return decodeUtf8Default(buffer, index, size);
      }
  
      /**
       * Decodes direct {@link ByteBuffer} instances into {@link String}.
       */
      abstract String decodeUtf8Direct(ByteBuffer buffer, int index, int size)
          throws InvalidProtocolBufferException;
  
      /**
       * Decodes {@link ByteBuffer} instances using the {@link ByteBuffer} API rather than
       * potentially faster approaches.
       */
      final String decodeUtf8Default(ByteBuffer buffer, int index, int size)
          throws InvalidProtocolBufferException {
        // Bitwise OR combines the sign bits so any negative value fails the check.
        if ((index | size | buffer.limit() - index - size) < 0) {
          throw new ArrayIndexOutOfBoundsException(
              String.format("buffer limit=%d, index=%d, limit=%d", buffer.limit(), index, size));
        }
  
        int offset = index;
        final int limit = offset + size;
  
        // The longest possible resulting String is the same as the number of input bytes, when it is
        // all ASCII. For other cases, this over-allocates and we will truncate in the end.
        char[] resultArr = new char[size];
        int resultPos = 0;
  
        // Optimize for 100% ASCII (Hotspot loves small simple top-level loops like this).
        // This simple loop stops when we encounter a byte >= 0x80 (i.e. non-ASCII).
        while (offset < limit) {
          byte b = buffer.get(offset);
          if (!DecodeUtil.isOneByte(b)) {
            break;
          }
          offset++;
          DecodeUtil.handleOneByte(b, resultArr, resultPos++);
        }
  
        while (offset < limit) {
          byte byte1 = buffer.get(offset++);
          if (DecodeUtil.isOneByte(byte1)) {
            DecodeUtil.handleOneByte(byte1, resultArr, resultPos++);
            // It's common for there to be multiple ASCII characters in a run mixed in, so add an
            // extra optimized loop to take care of these runs.
            while (offset < limit) {
              byte b = buffer.get(offset);
              if (!DecodeUtil.isOneByte(b)) {
                break;
              }
              offset++;
              DecodeUtil.handleOneByte(b, resultArr, resultPos++);
            }
          } else if (DecodeUtil.isTwoBytes(byte1)) {
            if (offset >= limit) {
              throw InvalidProtocolBufferException.invalidUtf8();
            }
            DecodeUtil.handleTwoBytes(
                byte1, /* byte2 */ buffer.get(offset++), resultArr, resultPos++);
          } else if (DecodeUtil.isThreeBytes(byte1)) {
            if (offset >= limit - 1) {
              throw InvalidProtocolBufferException.invalidUtf8();
            }
            DecodeUtil.handleThreeBytes(
                byte1,
                /* byte2 */ buffer.get(offset++),
                /* byte3 */ buffer.get(offset++),
                resultArr,
                resultPos++);
          } else {
            if (offset >= limit - 2) {
              throw InvalidProtocolBufferException.invalidUtf8();
            }
            DecodeUtil.handleFourBytes(
                byte1,
                /* byte2 */ buffer.get(offset++),
                /* byte3 */ buffer.get(offset++),
                /* byte4 */ buffer.get(offset++),
                resultArr,
                resultPos++);
            // 4-byte case requires two chars.
            resultPos++;
          }
        }
  
        return new String(resultArr, 0, resultPos);
      }
  
      /**
       * Encodes an input character sequence ({@code in}) to UTF-8 in the target array ({@code out}).
       * For a string, this method is similar to
       * <pre>{@code
       * byte[] a = string.getBytes(UTF_8);
       * System.arraycopy(a, 0, bytes, offset, a.length);
       * return offset + a.length;
       * }</pre>
       *
       * but is more efficient in both time and space. One key difference is that this method
       * requires paired surrogates, and therefore does not support chunking.
       * While {@code String.getBytes(UTF_8)} replaces unpaired surrogates with the default
       * replacement character, this method throws {@link UnpairedSurrogateException}.
       *
       * <p>To ensure sufficient space in the output buffer, either call {@link #encodedLength} to
       * compute the exact amount needed, or leave room for 
       * {@code Utf8.MAX_BYTES_PER_CHAR * sequence.length()}, which is the largest possible number
       * of bytes that any input can be encoded to.
       *
       * @param in the input character sequence to be encoded
       * @param out the target array
       * @param offset the starting offset in {@code bytes} to start writing at
       * @param length the length of the {@code bytes}, starting from {@code offset}
       * @throws UnpairedSurrogateException if {@code sequence} contains ill-formed UTF-16 (unpaired
       *     surrogates)
       * @throws ArrayIndexOutOfBoundsException if {@code sequence} encoded in UTF-8 is longer than
       *     {@code bytes.length - offset}
       * @return the new offset, equivalent to {@code offset + Utf8.encodedLength(sequence)}
       */
      abstract int encodeUtf8(CharSequence in, byte[] out, int offset, int length);
  
      /**
       * Encodes an input character sequence ({@code in}) to UTF-8 in the target buffer ({@code out}).
       * Upon returning from this method, the {@code out} position will point to the position after
       * the last encoded byte. This method requires paired surrogates, and therefore does not
       * support chunking.
       *
       * <p>To ensure sufficient space in the output buffer, either call {@link #encodedLength} to
       * compute the exact amount needed, or leave room for
       * {@code Utf8.MAX_BYTES_PER_CHAR * in.length()}, which is the largest possible number
       * of bytes that any input can be encoded to.
       *
       * @param in the source character sequence to be encoded
       * @param out the target buffer
       * @throws UnpairedSurrogateException if {@code in} contains ill-formed UTF-16 (unpaired
       *     surrogates)
       * @throws ArrayIndexOutOfBoundsException if {@code in} encoded in UTF-8 is longer than
       *     {@code out.remaining()}
       */
      final void encodeUtf8(CharSequence in, ByteBuffer out) {
        if (out.hasArray()) {
          final int offset = out.arrayOffset();
          int endIndex =
              Utf8.encode(in, out.array(), offset + out.position(), out.remaining());
          out.position(endIndex - offset);
        } else if (out.isDirect()) {
          encodeUtf8Direct(in, out);
        } else {
          encodeUtf8Default(in, out);
        }
      }
  
      /**
       * Encodes the input character sequence to a direct {@link ByteBuffer} instance.
       */
      abstract void encodeUtf8Direct(CharSequence in, ByteBuffer out);
  
      /**
       * Encodes the input character sequence to a {@link ByteBuffer} instance using the {@link
       * ByteBuffer} API, rather than potentially faster approaches.
       */
      final void encodeUtf8Default(CharSequence in, ByteBuffer out) {
        final int inLength = in.length();
        int outIx = out.position();
        int inIx = 0;
  
        // Since ByteBuffer.putXXX() already checks boundaries for us, no need to explicitly check
        // access. Assume the buffer is big enough and let it handle the out of bounds exception
        // if it occurs.
        try {
          // Designed to take advantage of
          // https://wikis.oracle.com/display/HotSpotInternals/RangeCheckElimination
          for (char c; inIx < inLength && (c = in.charAt(inIx)) < 0x80; ++inIx) {
            out.put(outIx + inIx, (byte) c);
          }
          if (inIx == inLength) {
            // Successfully encoded the entire string.
            out.position(outIx + inIx);
            return;
          }
  
          outIx += inIx;
          for (char c; inIx < inLength; ++inIx, ++outIx) {
            c = in.charAt(inIx);
            if (c < 0x80) {
              // One byte (0xxx xxxx)
              out.put(outIx, (byte) c);
            } else if (c < 0x800) {
              // Two bytes (110x xxxx 10xx xxxx)
  
              // Benchmarks show put performs better than putShort here (for HotSpot).
              out.put(outIx++, (byte) (0xC0 | (c >>> 6)));
              out.put(outIx, (byte) (0x80 | (0x3F & c)));
            } else if (c < MIN_SURROGATE || MAX_SURROGATE < c) {
              // Three bytes (1110 xxxx 10xx xxxx 10xx xxxx)
              // Maximum single-char code point is 0xFFFF, 16 bits.
  
              // Benchmarks show put performs better than putShort here (for HotSpot).
              out.put(outIx++, (byte) (0xE0 | (c >>> 12)));
              out.put(outIx++, (byte) (0x80 | (0x3F & (c >>> 6))));
              out.put(outIx, (byte) (0x80 | (0x3F & c)));
            } else {
              // Four bytes (1111 xxxx 10xx xxxx 10xx xxxx 10xx xxxx)
  
              // Minimum code point represented by a surrogate pair is 0x10000, 17 bits, four UTF-8
              // bytes
              final char low;
              if (inIx + 1 == inLength || !isSurrogatePair(c, (low = in.charAt(++inIx)))) {
                throw new UnpairedSurrogateException(inIx, inLength);
              }
              // TODO(nathanmittler): Consider using putInt() to improve performance.
              int codePoint = toCodePoint(c, low);
              out.put(outIx++, (byte) ((0xF << 4) | (codePoint >>> 18)));
              out.put(outIx++, (byte) (0x80 | (0x3F & (codePoint >>> 12))));
              out.put(outIx++, (byte) (0x80 | (0x3F & (codePoint >>> 6))));
              out.put(outIx, (byte) (0x80 | (0x3F & codePoint)));
            }
          }
  
          // Successfully encoded the entire string.
          out.position(outIx);
        } catch (IndexOutOfBoundsException e) {
          // TODO(nathanmittler): Consider making the API throw IndexOutOfBoundsException instead.
  
          // If we failed in the outer ASCII loop, outIx will not have been updated. In this case,
          // use inIx to determine the bad write index.
          int badWriteIndex = out.position() + Math.max(inIx, outIx - out.position() + 1);
          throw new ArrayIndexOutOfBoundsException(
              "Failed writing " + in.charAt(inIx) + " at index " + badWriteIndex);
        }
      }
    }
  
    /**
     * {@link Processor} implementation that does not use any {@code sun.misc.Unsafe} methods.
     */
    static final class SafeProcessor extends Processor {
      @Override
      int partialIsValidUtf8(int state, byte[] bytes, int index, int limit) {
        if (state != COMPLETE) {
          // The previous decoding operation was incomplete (or malformed).
          // We look for a well-formed sequence consisting of bytes from
          // the previous decoding operation (stored in state) together
          // with bytes from the array slice.
          //
          // We expect such "straddler characters" to be rare.
  
          if (index >= limit) {  // No bytes? No progress.
            return state;
          }
          int byte1 = (byte) state;
          // byte1 is never ASCII.
          if (byte1 < (byte) 0xE0) {
            // two-byte form
  
            // Simultaneously checks for illegal trailing-byte in
            // leading position and overlong 2-byte form.
            if (byte1 < (byte) 0xC2
                // byte2 trailing-byte test
                || bytes[index++] > (byte) 0xBF) {
              return MALFORMED;
            }
          } else if (byte1 < (byte) 0xF0) {
            // three-byte form
  
            // Get byte2 from saved state or array
            int byte2 = (byte) ~(state >> 8);
            if (byte2 == 0) {
              byte2 = bytes[index++];
              if (index >= limit) {
                return incompleteStateFor(byte1, byte2);
              }
            }
            if (byte2 > (byte) 0xBF
                // overlong? 5 most significant bits must not all be zero
                || (byte1 == (byte) 0xE0 && byte2 < (byte) 0xA0)
                // illegal surrogate codepoint?
                || (byte1 == (byte) 0xED && byte2 >= (byte) 0xA0)
                // byte3 trailing-byte test
                || bytes[index++] > (byte) 0xBF) {
              return MALFORMED;
            }
          } else {
            // four-byte form
  
            // Get byte2 and byte3 from saved state or array
            int byte2 = (byte) ~(state >> 8);
            int byte3 = 0;
            if (byte2 == 0) {
              byte2 = bytes[index++];
              if (index >= limit) {
                return incompleteStateFor(byte1, byte2);
              }
            } else {
              byte3 = (byte) (state >> 16);
            }
            if (byte3 == 0) {
              byte3 = bytes[index++];
              if (index >= limit) {
                return incompleteStateFor(byte1, byte2, byte3);
              }
            }
  
            // If we were called with state == MALFORMED, then byte1 is 0xFF,
            // which never occurs in well-formed UTF-8, and so we will return
            // MALFORMED again below.
  
            if (byte2 > (byte) 0xBF
                // Check that 1 <= plane <= 16.  Tricky optimized form of:
                // if (byte1 > (byte) 0xF4 ||
                //     byte1 == (byte) 0xF0 && byte2 < (byte) 0x90 ||
                //     byte1 == (byte) 0xF4 && byte2 > (byte) 0x8F)
                || (((byte1 << 28) + (byte2 - (byte) 0x90)) >> 30) != 0
                // byte3 trailing-byte test
                || byte3 > (byte) 0xBF
                // byte4 trailing-byte test
                || bytes[index++] > (byte) 0xBF) {
              return MALFORMED;
            }
          }
        }
  
        return partialIsValidUtf8(bytes, index, limit);
      }
  
      @Override
      int partialIsValidUtf8Direct(int state, ByteBuffer buffer, int index, int limit) {
        // For safe processing, we have to use the ByteBuffer API.
        return partialIsValidUtf8Default(state, buffer, index, limit);
      }
  
      @Override
      String decodeUtf8(byte[] bytes, int index, int size) throws InvalidProtocolBufferException {
        // Bitwise OR combines the sign bits so any negative value fails the check.
        if ((index | size | bytes.length - index - size) < 0) {
          throw new ArrayIndexOutOfBoundsException(
              String.format("buffer length=%d, index=%d, size=%d", bytes.length, index, size));
        }
  
        int offset = index;
        final int limit = offset + size;
  
        // The longest possible resulting String is the same as the number of input bytes, when it is
        // all ASCII. For other cases, this over-allocates and we will truncate in the end.
        char[] resultArr = new char[size];
        int resultPos = 0;
  
        // Optimize for 100% ASCII (Hotspot loves small simple top-level loops like this).
        // This simple loop stops when we encounter a byte >= 0x80 (i.e. non-ASCII).
        while (offset < limit) {
          byte b = bytes[offset];
          if (!DecodeUtil.isOneByte(b)) {
            break;
          }
          offset++;
          DecodeUtil.handleOneByte(b, resultArr, resultPos++);
        }
  
        while (offset < limit) {
          byte byte1 = bytes[offset++];
          if (DecodeUtil.isOneByte(byte1)) {
            DecodeUtil.handleOneByte(byte1, resultArr, resultPos++);
            // It's common for there to be multiple ASCII characters in a run mixed in, so add an
            // extra optimized loop to take care of these runs.
            while (offset < limit) {
              byte b = bytes[offset];
              if (!DecodeUtil.isOneByte(b)) {
                break;
              }
              offset++;
              DecodeUtil.handleOneByte(b, resultArr, resultPos++);
            }
          } else if (DecodeUtil.isTwoBytes(byte1)) {
            if (offset >= limit) {
              throw InvalidProtocolBufferException.invalidUtf8();
            }
            DecodeUtil.handleTwoBytes(byte1, /* byte2 */ bytes[offset++], resultArr, resultPos++);
          } else if (DecodeUtil.isThreeBytes(byte1)) {
            if (offset >= limit - 1) {
              throw InvalidProtocolBufferException.invalidUtf8();
            }
            DecodeUtil.handleThreeBytes(
                byte1,
                /* byte2 */ bytes[offset++],
                /* byte3 */ bytes[offset++],
                resultArr,
                resultPos++);
          } else {
            if (offset >= limit - 2) {
              throw InvalidProtocolBufferException.invalidUtf8();
            }
            DecodeUtil.handleFourBytes(
                byte1,
                /* byte2 */ bytes[offset++],
                /* byte3 */ bytes[offset++],
                /* byte4 */ bytes[offset++],
                resultArr,
                resultPos++);
            // 4-byte case requires two chars.
            resultPos++;
          }
        }
  
        return new String(resultArr, 0, resultPos);
      }
  
      @Override
      String decodeUtf8Direct(ByteBuffer buffer, int index, int size)
          throws InvalidProtocolBufferException {
        // For safe processing, we have to use the ByteBufferAPI.
        return decodeUtf8Default(buffer, index, size);
      }
  
      @Override
      int encodeUtf8(CharSequence in, byte[] out, int offset, int length) {
        int utf16Length = in.length();
        int j = offset;
        int i = 0;
        int limit = offset + length;
        // Designed to take advantage of
        // https://wikis.oracle.com/display/HotSpotInternals/RangeCheckElimination
        for (char c; i < utf16Length && i + j < limit && (c = in.charAt(i)) < 0x80; i++) {
          out[j + i] = (byte) c;
        }
        if (i == utf16Length) {
          return j + utf16Length;
        }
        j += i;
        for (char c; i < utf16Length; i++) {
          c = in.charAt(i);
          if (c < 0x80 && j < limit) {
            out[j++] = (byte) c;
          } else if (c < 0x800 && j <= limit - 2) { // 11 bits, two UTF-8 bytes
            out[j++] = (byte) ((0xF << 6) | (c >>> 6));
            out[j++] = (byte) (0x80 | (0x3F & c));
          } else if ((c < Character.MIN_SURROGATE || Character.MAX_SURROGATE < c) && j <= limit - 3) {
            // Maximum single-char code point is 0xFFFF, 16 bits, three UTF-8 bytes
            out[j++] = (byte) ((0xF << 5) | (c >>> 12));
            out[j++] = (byte) (0x80 | (0x3F & (c >>> 6)));
            out[j++] = (byte) (0x80 | (0x3F & c));
          } else if (j <= limit - 4) {
            // Minimum code point represented by a surrogate pair is 0x10000, 17 bits,
            // four UTF-8 bytes
            final char low;
            if (i + 1 == in.length()
                    || !Character.isSurrogatePair(c, (low = in.charAt(++i)))) {
              throw new UnpairedSurrogateException((i - 1), utf16Length);
            }
            int codePoint = Character.toCodePoint(c, low);
            out[j++] = (byte) ((0xF << 4) | (codePoint >>> 18));
            out[j++] = (byte) (0x80 | (0x3F & (codePoint >>> 12)));
            out[j++] = (byte) (0x80 | (0x3F & (codePoint >>> 6)));
            out[j++] = (byte) (0x80 | (0x3F & codePoint));
          } else {
            // If we are surrogates and we're not a surrogate pair, always throw an
            // UnpairedSurrogateException instead of an ArrayOutOfBoundsException.
            if ((Character.MIN_SURROGATE <= c && c <= Character.MAX_SURROGATE)
                && (i + 1 == in.length()
                    || !Character.isSurrogatePair(c, in.charAt(i + 1)))) {
              throw new UnpairedSurrogateException(i, utf16Length);
            }
            throw new ArrayIndexOutOfBoundsException("Failed writing " + c + " at index " + j);
          }
        }
        return j;
      }
  
      @Override
      void encodeUtf8Direct(CharSequence in, ByteBuffer out) {
        // For safe processing, we have to use the ByteBuffer API.
        encodeUtf8Default(in, out);
      }
  
      private static int partialIsValidUtf8(byte[] bytes, int index, int limit) {
        // Optimize for 100% ASCII (Hotspot loves small simple top-level loops like this).
        // This simple loop stops when we encounter a byte >= 0x80 (i.e. non-ASCII).
        while (index < limit && bytes[index] >= 0) {
          index++;
        }
  
        return (index >= limit) ? COMPLETE : partialIsValidUtf8NonAscii(bytes, index, limit);
      }
  
      private static int partialIsValidUtf8NonAscii(byte[] bytes, int index, int limit) {
        for (;;) {
          int byte1, byte2;
  
          // Optimize for interior runs of ASCII bytes.
          do {
            if (index >= limit) {
              return COMPLETE;
            }
          } while ((byte1 = bytes[index++]) >= 0);
  
          if (byte1 < (byte) 0xE0) {
            // two-byte form
  
            if (index >= limit) {
              // Incomplete sequence
              return byte1;
            }
  
            // Simultaneously checks for illegal trailing-byte in
            // leading position and overlong 2-byte form.
            if (byte1 < (byte) 0xC2
                || bytes[index++] > (byte) 0xBF) {
              return MALFORMED;
            }
          } else if (byte1 < (byte) 0xF0) {
            // three-byte form
  
            if (index >= limit - 1) { // incomplete sequence
              return incompleteStateFor(bytes, index, limit);
            }
            if ((byte2 = bytes[index++]) > (byte) 0xBF
                // overlong? 5 most significant bits must not all be zero
                || (byte1 == (byte) 0xE0 && byte2 < (byte) 0xA0)
                // check for illegal surrogate codepoints
                || (byte1 == (byte) 0xED && byte2 >= (byte) 0xA0)
                // byte3 trailing-byte test
                || bytes[index++] > (byte) 0xBF) {
              return MALFORMED;
            }
          } else {
            // four-byte form
  
            if (index >= limit - 2) {  // incomplete sequence
              return incompleteStateFor(bytes, index, limit);
            }
            if ((byte2 = bytes[index++]) > (byte) 0xBF
                // Check that 1 <= plane <= 16.  Tricky optimized form of:
                // if (byte1 > (byte) 0xF4 ||
                //     byte1 == (byte) 0xF0 && byte2 < (byte) 0x90 ||
                //     byte1 == (byte) 0xF4 && byte2 > (byte) 0x8F)
                || (((byte1 << 28) + (byte2 - (byte) 0x90)) >> 30) != 0
                // byte3 trailing-byte test
                || bytes[index++] > (byte) 0xBF
                // byte4 trailing-byte test
                || bytes[index++] > (byte) 0xBF) {
              return MALFORMED;
            }
          }
        }
      }
    }
  
    /**
     * {@link Processor} that uses {@code sun.misc.Unsafe} where possible to improve performance.
     */
    static final class UnsafeProcessor extends Processor {
      /**
       * Indicates whether or not all required unsafe operations are supported on this platform.
       */
      static boolean isAvailable() {
        return hasUnsafeArrayOperations() && hasUnsafeByteBufferOperations();
      }
  
      @Override
      int partialIsValidUtf8(int state, byte[] bytes, final int index, final int limit) {
        // Bitwise OR combines the sign bits so any negative value fails the check.
        if ((index | limit | bytes.length - limit) < 0) {
          throw new ArrayIndexOutOfBoundsException(
              String.format("Array length=%d, index=%d, limit=%d", bytes.length, index, limit));
        }
        long offset = index;
        final long offsetLimit = limit;
        if (state != COMPLETE) {
          // The previous decoding operation was incomplete (or malformed).
          // We look for a well-formed sequence consisting of bytes from
          // the previous decoding operation (stored in state) together
          // with bytes from the array slice.
          //
          // We expect such "straddler characters" to be rare.
  
          if (offset >= offsetLimit) {  // No bytes? No progress.
            return state;
          }
          int byte1 = (byte) state;
          // byte1 is never ASCII.
          if (byte1 < (byte) 0xE0) {
            // two-byte form
  
            // Simultaneously checks for illegal trailing-byte in
            // leading position and overlong 2-byte form.
            if (byte1 < (byte) 0xC2
                // byte2 trailing-byte test
                || UnsafeUtil.getByte(bytes, offset++) > (byte) 0xBF) {
              return MALFORMED;
            }
          } else if (byte1 < (byte) 0xF0) {
            // three-byte form
  
            // Get byte2 from saved state or array
            int byte2 = (byte) ~(state >> 8);
            if (byte2 == 0) {
              byte2 = UnsafeUtil.getByte(bytes, offset++);
              if (offset >= offsetLimit) {
                return incompleteStateFor(byte1, byte2);
              }
            }
            if (byte2 > (byte) 0xBF
                // overlong? 5 most significant bits must not all be zero
                || (byte1 == (byte) 0xE0 && byte2 < (byte) 0xA0)
                // illegal surrogate codepoint?
                || (byte1 == (byte) 0xED && byte2 >= (byte) 0xA0)
                // byte3 trailing-byte test
                || UnsafeUtil.getByte(bytes, offset++) > (byte) 0xBF) {
              return MALFORMED;
            }
          } else {
            // four-byte form
  
            // Get byte2 and byte3 from saved state or array
            int byte2 = (byte) ~(state >> 8);
            int byte3 = 0;
            if (byte2 == 0) {
              byte2 = UnsafeUtil.getByte(bytes, offset++);
              if (offset >= offsetLimit) {
                return incompleteStateFor(byte1, byte2);
              }
            } else {
              byte3 = (byte) (state >> 16);
            }
            if (byte3 == 0) {
              byte3 = UnsafeUtil.getByte(bytes, offset++);
              if (offset >= offsetLimit) {
                return incompleteStateFor(byte1, byte2, byte3);
              }
            }
  
            // If we were called with state == MALFORMED, then byte1 is 0xFF,
            // which never occurs in well-formed UTF-8, and so we will return
            // MALFORMED again below.
  
            if (byte2 > (byte) 0xBF
                // Check that 1 <= plane <= 16.  Tricky optimized form of:
                // if (byte1 > (byte) 0xF4 ||
                //     byte1 == (byte) 0xF0 && byte2 < (byte) 0x90 ||
                //     byte1 == (byte) 0xF4 && byte2 > (byte) 0x8F)
                || (((byte1 << 28) + (byte2 - (byte) 0x90)) >> 30) != 0
                // byte3 trailing-byte test
                || byte3 > (byte) 0xBF
                // byte4 trailing-byte test
                || UnsafeUtil.getByte(bytes, offset++) > (byte) 0xBF) {
              return MALFORMED;
            }
          }
        }
  
        return partialIsValidUtf8(bytes, offset, (int) (offsetLimit - offset));
      }
  
      @Override
      int partialIsValidUtf8Direct(
          final int state, ByteBuffer buffer, final int index, final int limit) {
        // Bitwise OR combines the sign bits so any negative value fails the check.
        if ((index | limit | buffer.limit() - limit) < 0) {
          throw new ArrayIndexOutOfBoundsException(
              String.format("buffer limit=%d, index=%d, limit=%d", buffer.limit(), index, limit));
        }
        long address = addressOffset(buffer) + index;
        final long addressLimit = address + (limit - index);
        if (state != COMPLETE) {
          // The previous decoding operation was incomplete (or malformed).
          // We look for a well-formed sequence consisting of bytes from
          // the previous decoding operation (stored in state) together
          // with bytes from the array slice.
          //
          // We expect such "straddler characters" to be rare.
  
          if (address >= addressLimit) { // No bytes? No progress.
            return state;
          }
  
          final int byte1 = (byte) state;
          // byte1 is never ASCII.
          if (byte1 < (byte) 0xE0) {
            // two-byte form
  
            // Simultaneously checks for illegal trailing-byte in
            // leading position and overlong 2-byte form.
            if (byte1 < (byte) 0xC2
                // byte2 trailing-byte test
                || UnsafeUtil.getByte(address++) > (byte) 0xBF) {
              return MALFORMED;
            }
          } else if (byte1 < (byte) 0xF0) {
            // three-byte form
  
            // Get byte2 from saved state or array
            int byte2 = (byte) ~(state >> 8);
            if (byte2 == 0) {
              byte2 = UnsafeUtil.getByte(address++);
              if (address >= addressLimit) {
                return incompleteStateFor(byte1, byte2);
              }
            }
            if (byte2 > (byte) 0xBF
                // overlong? 5 most significant bits must not all be zero
                || (byte1 == (byte) 0xE0 && byte2 < (byte) 0xA0)
                // illegal surrogate codepoint?
                || (byte1 == (byte) 0xED && byte2 >= (byte) 0xA0)
                // byte3 trailing-byte test
                || UnsafeUtil.getByte(address++) > (byte) 0xBF) {
              return MALFORMED;
            }
          } else {
            // four-byte form
  
            // Get byte2 and byte3 from saved state or array
            int byte2 = (byte) ~(state >> 8);
            int byte3 = 0;
            if (byte2 == 0) {
              byte2 = UnsafeUtil.getByte(address++);
              if (address >= addressLimit) {
                return incompleteStateFor(byte1, byte2);
              }
            } else {
              byte3 = (byte) (state >> 16);
            }
            if (byte3 == 0) {
              byte3 = UnsafeUtil.getByte(address++);
              if (address >= addressLimit) {
                return incompleteStateFor(byte1, byte2, byte3);
              }
            }
  
            // If we were called with state == MALFORMED, then byte1 is 0xFF,
            // which never occurs in well-formed UTF-8, and so we will return
            // MALFORMED again below.
  
            if (byte2 > (byte) 0xBF
                // Check that 1 <= plane <= 16.  Tricky optimized form of:
                // if (byte1 > (byte) 0xF4 ||
                //     byte1 == (byte) 0xF0 && byte2 < (byte) 0x90 ||
                //     byte1 == (byte) 0xF4 && byte2 > (byte) 0x8F)
                || (((byte1 << 28) + (byte2 - (byte) 0x90)) >> 30) != 0
                // byte3 trailing-byte test
                || byte3 > (byte) 0xBF
                // byte4 trailing-byte test
                || UnsafeUtil.getByte(address++) > (byte) 0xBF) {
              return MALFORMED;
            }
          }
        }
  
        return partialIsValidUtf8(address, (int) (addressLimit - address));
      }
  
      @Override
      String decodeUtf8(byte[] bytes, int index, int size) throws InvalidProtocolBufferException {
        if ((index | size | bytes.length - index - size) < 0) {
          throw new ArrayIndexOutOfBoundsException(
              String.format("buffer length=%d, index=%d, size=%d", bytes.length, index, size));
        }
  
        int offset = index;
        final int limit = offset + size;
  
        // The longest possible resulting String is the same as the number of input bytes, when it is
        // all ASCII. For other cases, this over-allocates and we will truncate in the end.
        char[] resultArr = new char[size];
        int resultPos = 0;
  
        // Optimize for 100% ASCII (Hotspot loves small simple top-level loops like this).
        // This simple loop stops when we encounter a byte >= 0x80 (i.e. non-ASCII).
        while (offset < limit) {
          byte b = UnsafeUtil.getByte(bytes, offset);
          if (!DecodeUtil.isOneByte(b)) {
            break;
          }
          offset++;
          DecodeUtil.handleOneByte(b, resultArr, resultPos++);
        }
  
        while (offset < limit) {
          byte byte1 = UnsafeUtil.getByte(bytes, offset++);
          if (DecodeUtil.isOneByte(byte1)) {
            DecodeUtil.handleOneByte(byte1, resultArr, resultPos++);
            // It's common for there to be multiple ASCII characters in a run mixed in, so add an
            // extra optimized loop to take care of these runs.
            while (offset < limit) {
              byte b = UnsafeUtil.getByte(bytes, offset);
              if (!DecodeUtil.isOneByte(b)) {
                break;
              }
              offset++;
              DecodeUtil.handleOneByte(b, resultArr, resultPos++);
            }
          } else if (DecodeUtil.isTwoBytes(byte1)) {
            if (offset >= limit) {
              throw InvalidProtocolBufferException.invalidUtf8();
            }
            DecodeUtil.handleTwoBytes(
                byte1, /* byte2 */ UnsafeUtil.getByte(bytes, offset++), resultArr, resultPos++);
          } else if (DecodeUtil.isThreeBytes(byte1)) {
            if (offset >= limit - 1) {
              throw InvalidProtocolBufferException.invalidUtf8();
            }
            DecodeUtil.handleThreeBytes(
                byte1,
                /* byte2 */ UnsafeUtil.getByte(bytes, offset++),
                /* byte3 */ UnsafeUtil.getByte(bytes, offset++),
                resultArr,
                resultPos++);
          } else {
            if (offset >= limit - 2) {
              throw InvalidProtocolBufferException.invalidUtf8();
            }
            DecodeUtil.handleFourBytes(
                byte1,
                /* byte2 */ UnsafeUtil.getByte(bytes, offset++),
                /* byte3 */ UnsafeUtil.getByte(bytes, offset++),
                /* byte4 */ UnsafeUtil.getByte(bytes, offset++),
                resultArr,
                resultPos++);
            // 4-byte case requires two chars.
            resultPos++;
          }
        }
  
        if (resultPos < resultArr.length) {
          resultArr = Arrays.copyOf(resultArr, resultPos);
        }
        return UnsafeUtil.moveToString(resultArr);
      }
  
      @Override
      String decodeUtf8Direct(ByteBuffer buffer, int index, int size)
          throws InvalidProtocolBufferException {
        // Bitwise OR combines the sign bits so any negative value fails the check.
        if ((index | size | buffer.limit() - index - size) < 0) {
          throw new ArrayIndexOutOfBoundsException(
              String.format("buffer limit=%d, index=%d, limit=%d", buffer.limit(), index, size));
        }
        long address = UnsafeUtil.addressOffset(buffer) + index;
        final long addressLimit = address + size;
  
        // The longest possible resulting String is the same as the number of input bytes, when it is
        // all ASCII. For other cases, this over-allocates and we will truncate in the end.
        char[] resultArr = new char[size];
        int resultPos = 0;
  
        // Optimize for 100% ASCII (Hotspot loves small simple top-level loops like this).
        // This simple loop stops when we encounter a byte >= 0x80 (i.e. non-ASCII).
        while (address < addressLimit) {
          byte b = UnsafeUtil.getByte(address);
          if (!DecodeUtil.isOneByte(b)) {
            break;
          }
          address++;
          DecodeUtil.handleOneByte(b, resultArr, resultPos++);
        }
  
        while (address < addressLimit) {
          byte byte1 = UnsafeUtil.getByte(address++);
          if (DecodeUtil.isOneByte(byte1)) {
            DecodeUtil.handleOneByte(byte1, resultArr, resultPos++);
            // It's common for there to be multiple ASCII characters in a run mixed in, so add an
            // extra optimized loop to take care of these runs.
            while (address < addressLimit) {
              byte b = UnsafeUtil.getByte(address);
              if (!DecodeUtil.isOneByte(b)) {
                break;
              }
              address++;
              DecodeUtil.handleOneByte(b, resultArr, resultPos++);
            }
          } else if (DecodeUtil.isTwoBytes(byte1)) {
            if (address >= addressLimit) {
              throw InvalidProtocolBufferException.invalidUtf8();
            }
            DecodeUtil.handleTwoBytes(
                byte1, /* byte2 */ UnsafeUtil.getByte(address++), resultArr, resultPos++);
          } else if (DecodeUtil.isThreeBytes(byte1)) {
            if (address >= addressLimit - 1) {
              throw InvalidProtocolBufferException.invalidUtf8();
            }
            DecodeUtil.handleThreeBytes(
                byte1,
                /* byte2 */ UnsafeUtil.getByte(address++),
                /* byte3 */ UnsafeUtil.getByte(address++),
                resultArr,
                resultPos++);
          } else {
            if (address >= addressLimit - 2) {
              throw InvalidProtocolBufferException.invalidUtf8();
            }
            DecodeUtil.handleFourBytes(
                byte1,
                /* byte2 */ UnsafeUtil.getByte(address++),
                /* byte3 */ UnsafeUtil.getByte(address++),
                /* byte4 */ UnsafeUtil.getByte(address++),
                resultArr,
                resultPos++);
            // 4-byte case requires two chars.
            resultPos++;
          }
        }
  
        if (resultPos < resultArr.length) {
          resultArr = Arrays.copyOf(resultArr, resultPos);
        }
        return UnsafeUtil.moveToString(resultArr);
      }
  
      @Override
      int encodeUtf8(final CharSequence in, final byte[] out, final int offset, final int length) {
        long outIx = offset;
        final long outLimit = outIx + length;
        final int inLimit = in.length();
        if (inLimit > length || out.length - length < offset) {
          // Not even enough room for an ASCII-encoded string.
          throw new ArrayIndexOutOfBoundsException(
              "Failed writing " + in.charAt(inLimit - 1) + " at index " + (offset + length));
        }
  
        // Designed to take advantage of
        // https://wikis.oracle.com/display/HotSpotInternals/RangeCheckElimination
        int inIx = 0;
        for (char c; inIx < inLimit && (c = in.charAt(inIx)) < 0x80; ++inIx) {
          UnsafeUtil.putByte(out, outIx++, (byte) c);
        }
        if (inIx == inLimit) {
          // We're done, it was ASCII encoded.
          return (int) outIx;
        }
  
        for (char c; inIx < inLimit; ++inIx) {
          c = in.charAt(inIx);
          if (c < 0x80 && outIx < outLimit) {
            UnsafeUtil.putByte(out, outIx++, (byte) c);
          } else if (c < 0x800 && outIx <= outLimit - 2L) { // 11 bits, two UTF-8 bytes
            UnsafeUtil.putByte(out, outIx++, (byte) ((0xF << 6) | (c >>> 6)));
            UnsafeUtil.putByte(out, outIx++, (byte) (0x80 | (0x3F & c)));
          } else if ((c < MIN_SURROGATE || MAX_SURROGATE < c) && outIx <= outLimit - 3L) {
            // Maximum single-char code point is 0xFFFF, 16 bits, three UTF-8 bytes
            UnsafeUtil.putByte(out, outIx++, (byte) ((0xF << 5) | (c >>> 12)));
            UnsafeUtil.putByte(out, outIx++, (byte) (0x80 | (0x3F & (c >>> 6))));
            UnsafeUtil.putByte(out, outIx++, (byte) (0x80 | (0x3F & c)));
          } else if (outIx <= outLimit - 4L) {
            // Minimum code point represented by a surrogate pair is 0x10000, 17 bits, four UTF-8
            // bytes
            final char low;
            if (inIx + 1 == inLimit || !isSurrogatePair(c, (low = in.charAt(++inIx)))) {
              throw new UnpairedSurrogateException((inIx - 1), inLimit);
            }
            int codePoint = toCodePoint(c, low);
            UnsafeUtil.putByte(out, outIx++, (byte) ((0xF << 4) | (codePoint >>> 18)));
            UnsafeUtil.putByte(out, outIx++, (byte) (0x80 | (0x3F & (codePoint >>> 12))));
            UnsafeUtil.putByte(out, outIx++, (byte) (0x80 | (0x3F & (codePoint >>> 6))));
            UnsafeUtil.putByte(out, outIx++, (byte) (0x80 | (0x3F & codePoint)));
          } else {
            if ((MIN_SURROGATE <= c && c <= MAX_SURROGATE)
                && (inIx + 1 == inLimit || !isSurrogatePair(c, in.charAt(inIx + 1)))) {
              // We are surrogates and we're not a surrogate pair.
              throw new UnpairedSurrogateException(inIx, inLimit);
            }
            // Not enough space in the output buffer.
            throw new ArrayIndexOutOfBoundsException("Failed writing " + c + " at index " + outIx);
          }
        }
  
        // All bytes have been encoded.
        return (int) outIx;
      }
  
      @Override
      void encodeUtf8Direct(CharSequence in, ByteBuffer out) {
        final long address = addressOffset(out);
        long outIx = address + out.position();
        final long outLimit = address + out.limit();
        final int inLimit = in.length();
        if (inLimit > outLimit - outIx) {
          // Not even enough room for an ASCII-encoded string.
          throw new ArrayIndexOutOfBoundsException(
              "Failed writing " + in.charAt(inLimit - 1) + " at index " + out.limit());
        }
  
        // Designed to take advantage of
        // https://wikis.oracle.com/display/HotSpotInternals/RangeCheckElimination
        int inIx = 0;
        for (char c; inIx < inLimit && (c = in.charAt(inIx)) < 0x80; ++inIx) {
          UnsafeUtil.putByte(outIx++, (byte) c);
        }
        if (inIx == inLimit) {
          // We're done, it was ASCII encoded.
          out.position((int) (outIx - address));
          return;
        }
  
        for (char c; inIx < inLimit; ++inIx) {
          c = in.charAt(inIx);
          if (c < 0x80 && outIx < outLimit) {
            UnsafeUtil.putByte(outIx++, (byte) c);
          } else if (c < 0x800 && outIx <= outLimit - 2L) { // 11 bits, two UTF-8 bytes
            UnsafeUtil.putByte(outIx++, (byte) ((0xF << 6) | (c >>> 6)));
            UnsafeUtil.putByte(outIx++, (byte) (0x80 | (0x3F & c)));
          } else if ((c < MIN_SURROGATE || MAX_SURROGATE < c) && outIx <= outLimit - 3L) {
            // Maximum single-char code point is 0xFFFF, 16 bits, three UTF-8 bytes
            UnsafeUtil.putByte(outIx++, (byte) ((0xF << 5) | (c >>> 12)));
            UnsafeUtil.putByte(outIx++, (byte) (0x80 | (0x3F & (c >>> 6))));
            UnsafeUtil.putByte(outIx++, (byte) (0x80 | (0x3F & c)));
          } else if (outIx <= outLimit - 4L) {
            // Minimum code point represented by a surrogate pair is 0x10000, 17 bits, four UTF-8
            // bytes
            final char low;
            if (inIx + 1 == inLimit || !isSurrogatePair(c, (low = in.charAt(++inIx)))) {
              throw new UnpairedSurrogateException((inIx - 1), inLimit);
            }
            int codePoint = toCodePoint(c, low);
            UnsafeUtil.putByte(outIx++, (byte) ((0xF << 4) | (codePoint >>> 18)));
            UnsafeUtil.putByte(outIx++, (byte) (0x80 | (0x3F & (codePoint >>> 12))));
            UnsafeUtil.putByte(outIx++, (byte) (0x80 | (0x3F & (codePoint >>> 6))));
            UnsafeUtil.putByte(outIx++, (byte) (0x80 | (0x3F & codePoint)));
          } else {
            if ((MIN_SURROGATE <= c && c <= MAX_SURROGATE)
                && (inIx + 1 == inLimit || !isSurrogatePair(c, in.charAt(inIx + 1)))) {
              // We are surrogates and we're not a surrogate pair.
              throw new UnpairedSurrogateException(inIx, inLimit);
            }
            // Not enough space in the output buffer.
            throw new ArrayIndexOutOfBoundsException("Failed writing " + c + " at index " + outIx);
          }
        }
  
        // All bytes have been encoded.
        out.position((int) (outIx - address));
      }
  
      /**
       * Counts (approximately) the number of consecutive ASCII characters starting from the given
       * position, using the most efficient method available to the platform.
       *
       * @param bytes the array containing the character sequence
       * @param offset the offset position of the index (same as index + arrayBaseOffset)
       * @param maxChars the maximum number of characters to count
       * @return the number of ASCII characters found. The stopping position will be at or
       * before the first non-ASCII byte.
       */
      private static int unsafeEstimateConsecutiveAscii(
          byte[] bytes, long offset, final int maxChars) {
        if (maxChars < UNSAFE_COUNT_ASCII_THRESHOLD) {
          // Don't bother with small strings.
          return 0;
        }
  
        for (int i = 0; i < maxChars; i++) {
          if (UnsafeUtil.getByte(bytes, offset++) < 0) {
            return i;
          }
        }
        return maxChars;
      }
  
      /**
       * Same as {@link Utf8#estimateConsecutiveAscii(ByteBuffer, int, int)} except that it uses the
       * most efficient method available to the platform.
       */
      private static int unsafeEstimateConsecutiveAscii(long address, final int maxChars) {
        int remaining = maxChars;
        if (remaining < UNSAFE_COUNT_ASCII_THRESHOLD) {
          // Don't bother with small strings.
          return 0;
        }
  
        // Read bytes until 8-byte aligned so that we can read longs in the loop below.
        // We do this by ANDing the address with 7 to determine the number of bytes that need to
        // be read before we're 8-byte aligned.
        final int unaligned = 8 - ((int) address & 7);
        for (int j = unaligned; j > 0; j--) {
          if (UnsafeUtil.getByte(address++) < 0) {
            return unaligned - j;
          }
        }
  
        // This simple loop stops when we encounter a byte >= 0x80 (i.e. non-ASCII).
        // To speed things up further, we're reading longs instead of bytes so we use a mask to
        // determine if any byte in the current long is non-ASCII.
        remaining -= unaligned;
        for (; remaining >= 8 && (UnsafeUtil.getLong(address) & ASCII_MASK_LONG) == 0;
            address += 8, remaining -= 8) {}
        return maxChars - remaining;
      }
  
      private static int partialIsValidUtf8(final byte[] bytes, long offset, int remaining) {
        // Skip past ASCII characters as quickly as possible. 
        final int skipped = unsafeEstimateConsecutiveAscii(bytes, offset, remaining);
        remaining -= skipped;
        offset += skipped;
  
        for (;;) {
          // Optimize for interior runs of ASCII bytes.
          // TODO(nathanmittler): Consider checking 8 bytes at a time after some threshold?
          // Maybe after seeing a few in a row that are ASCII, go back to fast mode?
          int byte1 = 0;
          for (; remaining > 0 && (byte1 = UnsafeUtil.getByte(bytes, offset++)) >= 0; --remaining) {
          }
          if (remaining == 0) {
            return COMPLETE;
          }
          remaining--;
  
          // If we're here byte1 is not ASCII. Only need to handle 2-4 byte forms.
          if (byte1 < (byte) 0xE0) {
            // Two-byte form (110xxxxx 10xxxxxx)
            if (remaining == 0) {
              // Incomplete sequence
              return byte1;
            }
            remaining--;
  
            // Simultaneously checks for illegal trailing-byte in
            // leading position and overlong 2-byte form.
            if (byte1 < (byte) 0xC2
                || UnsafeUtil.getByte(bytes, offset++) > (byte) 0xBF) {
              return MALFORMED;
            }
          } else if (byte1 < (byte) 0xF0) {
            // Three-byte form (1110xxxx 10xxxxxx 10xxxxxx)
            if (remaining < 2) {
              // Incomplete sequence
              return unsafeIncompleteStateFor(bytes, byte1, offset, remaining);
            }
            remaining -= 2;
  
            final int byte2;
            if ((byte2 = UnsafeUtil.getByte(bytes, offset++)) > (byte) 0xBF
                // overlong? 5 most significant bits must not all be zero
                || (byte1 == (byte) 0xE0 && byte2 < (byte) 0xA0)
                // check for illegal surrogate codepoints
                || (byte1 == (byte) 0xED && byte2 >= (byte) 0xA0)
                // byte3 trailing-byte test
                || UnsafeUtil.getByte(bytes, offset++) > (byte) 0xBF) {
              return MALFORMED;
            }
          } else {
            // Four-byte form (1110xxxx 10xxxxxx 10xxxxxx 10xxxxxx)
            if (remaining < 3) {
              // Incomplete sequence
              return unsafeIncompleteStateFor(bytes, byte1, offset, remaining);
            }
            remaining -= 3;
  
            final int byte2;
            if ((byte2 = UnsafeUtil.getByte(bytes, offset++)) > (byte) 0xBF
                // Check that 1 <= plane <= 16.  Tricky optimized form of:
                // if (byte1 > (byte) 0xF4 ||
                //     byte1 == (byte) 0xF0 && byte2 < (byte) 0x90 ||
                //     byte1 == (byte) 0xF4 && byte2 > (byte) 0x8F)
                || (((byte1 << 28) + (byte2 - (byte) 0x90)) >> 30) != 0
                // byte3 trailing-byte test
                || UnsafeUtil.getByte(bytes, offset++) > (byte) 0xBF
                // byte4 trailing-byte test
                || UnsafeUtil.getByte(bytes, offset++) > (byte) 0xBF) {
              return MALFORMED;
            }
          }
        }
      }
  
      private static int partialIsValidUtf8(long address, int remaining) {
        // Skip past ASCII characters as quickly as possible.
        final int skipped = unsafeEstimateConsecutiveAscii(address, remaining);
        address += skipped;
        remaining -= skipped;
  
        for (;;) {
          // Optimize for interior runs of ASCII bytes.
          // TODO(nathanmittler): Consider checking 8 bytes at a time after some threshold?
          // Maybe after seeing a few in a row that are ASCII, go back to fast mode?
          int byte1 = 0;
          for (; remaining > 0 && (byte1 = UnsafeUtil.getByte(address++)) >= 0; --remaining) {
          }
          if (remaining == 0) {
            return COMPLETE;
          }
          remaining--;
  
          if (byte1 < (byte) 0xE0) {
            // Two-byte form
  
            if (remaining == 0) {
              // Incomplete sequence
              return byte1;
            }
            remaining--;
  
            // Simultaneously checks for illegal trailing-byte in
            // leading position and overlong 2-byte form.
            if (byte1 < (byte) 0xC2 || UnsafeUtil.getByte(address++) > (byte) 0xBF) {
              return MALFORMED;
            }
          } else if (byte1 < (byte) 0xF0) {
            // Three-byte form
  
            if (remaining < 2) {
              // Incomplete sequence
              return unsafeIncompleteStateFor(address, byte1, remaining);
            }
            remaining -= 2;
  
            final byte byte2 = UnsafeUtil.getByte(address++);
            if (byte2 > (byte) 0xBF
                // overlong? 5 most significant bits must not all be zero
                || (byte1 == (byte) 0xE0 && byte2 < (byte) 0xA0)
                // check for illegal surrogate codepoints
                || (byte1 == (byte) 0xED && byte2 >= (byte) 0xA0)
                // byte3 trailing-byte test
                || UnsafeUtil.getByte(address++) > (byte) 0xBF) {
              return MALFORMED;
            }
          } else {
            // Four-byte form
  
            if (remaining < 3) {
              // Incomplete sequence
              return unsafeIncompleteStateFor(address, byte1, remaining);
            }
            remaining -= 3;
  
            final byte byte2 = UnsafeUtil.getByte(address++);
            if (byte2 > (byte) 0xBF
                // Check that 1 <= plane <= 16.  Tricky optimized form of:
                // if (byte1 > (byte) 0xF4 ||
                //     byte1 == (byte) 0xF0 && byte2 < (byte) 0x90 ||
                //     byte1 == (byte) 0xF4 && byte2 > (byte) 0x8F)
                || (((byte1 << 28) + (byte2 - (byte) 0x90)) >> 30) != 0
                // byte3 trailing-byte test
                || UnsafeUtil.getByte(address++) > (byte) 0xBF
                // byte4 trailing-byte test
                || UnsafeUtil.getByte(address++) > (byte) 0xBF) {
              return MALFORMED;
            }
          }
        }
      }
  
      private static int unsafeIncompleteStateFor(byte[] bytes, int byte1, long offset,
          int remaining) {
        switch (remaining) {
          case 0: {
            return incompleteStateFor(byte1);
          }
          case 1: {
            return incompleteStateFor(byte1, UnsafeUtil.getByte(bytes, offset));
          }
          case 2: {
            return incompleteStateFor(byte1, UnsafeUtil.getByte(bytes, offset),
                UnsafeUtil.getByte(bytes, offset + 1));
          }
          default: {
            throw new AssertionError();
          }
        }
      }
  
      private static int unsafeIncompleteStateFor(long address, final int byte1, int remaining) {
        switch (remaining) {
          case 0: {
            return incompleteStateFor(byte1);
          }
          case 1: {
            return incompleteStateFor(byte1, UnsafeUtil.getByte(address));
          }
          case 2: {
            return incompleteStateFor(byte1, UnsafeUtil.getByte(address),
                UnsafeUtil.getByte(address + 1));
          }
          default: {
            throw new AssertionError();
          }
        }
      }
    }
  
    /**
     * Utility methods for decoding bytes into {@link String}. Callers are responsible for extracting
     * bytes (possibly using Unsafe methods), and checking remaining bytes. All other UTF-8 validity
     * checks and codepoint conversion happen in this class.
     */
    private static class DecodeUtil {
  
      /**
       * Returns whether this is a single-byte codepoint (i.e., ASCII) with the form '0XXXXXXX'.
       */
      private static boolean isOneByte(byte b) {
        return b >= 0;
      }
  
      /**
       * Returns whether this is a two-byte codepoint with the form '10XXXXXX'.
       */
      private static boolean isTwoBytes(byte b) {
        return b < (byte) 0xE0;
      }
  
      /**
       * Returns whether this is a three-byte codepoint with the form '110XXXXX'.
       */
      private static boolean isThreeBytes(byte b) {
        return b < (byte) 0xF0;
      }
  
      private static void handleOneByte(byte byte1, char[] resultArr, int resultPos) {
        resultArr[resultPos] = (char) byte1;
      }
  
      private static void handleTwoBytes(
          byte byte1, byte byte2, char[] resultArr, int resultPos)
          throws InvalidProtocolBufferException {
        // Simultaneously checks for illegal trailing-byte in leading position (<= '11000000') and
        // overlong 2-byte, '11000001'.
        if (byte1 < (byte) 0xC2
            || isNotTrailingByte(byte2)) {
          throw InvalidProtocolBufferException.invalidUtf8();
        }
        resultArr[resultPos] = (char) (((byte1 & 0x1F) << 6) | trailingByteValue(byte2));
      }
  
      private static void handleThreeBytes(
          byte byte1, byte byte2, byte byte3, char[] resultArr, int resultPos)
          throws InvalidProtocolBufferException {
        if (isNotTrailingByte(byte2)
            // overlong? 5 most significant bits must not all be zero
            || (byte1 == (byte) 0xE0 && byte2 < (byte) 0xA0)
            // check for illegal surrogate codepoints
            || (byte1 == (byte) 0xED && byte2 >= (byte) 0xA0)
            || isNotTrailingByte(byte3)) {
          throw InvalidProtocolBufferException.invalidUtf8();
        }
        resultArr[resultPos] = (char)
            (((byte1 & 0x0F) << 12) | (trailingByteValue(byte2) << 6) | trailingByteValue(byte3));
      }
  
      private static void handleFourBytes(
          byte byte1, byte byte2, byte byte3, byte byte4, char[] resultArr, int resultPos)
          throws InvalidProtocolBufferException{
        if (isNotTrailingByte(byte2)
            // Check that 1 <= plane <= 16.  Tricky optimized form of:
            //   valid 4-byte leading byte?
            // if (byte1 > (byte) 0xF4 ||
            //   overlong? 4 most significant bits must not all be zero
            //     byte1 == (byte) 0xF0 && byte2 < (byte) 0x90 ||
            //   codepoint larger than the highest code point (U+10FFFF)?
            //     byte1 == (byte) 0xF4 && byte2 > (byte) 0x8F)
            || (((byte1 << 28) + (byte2 - (byte) 0x90)) >> 30) != 0
            || isNotTrailingByte(byte3)
            || isNotTrailingByte(byte4)) {
          throw InvalidProtocolBufferException.invalidUtf8();
        }
        int codepoint = ((byte1 & 0x07) << 18)
            | (trailingByteValue(byte2) << 12)
            | (trailingByteValue(byte3) << 6)
            | trailingByteValue(byte4);
        resultArr[resultPos] = DecodeUtil.highSurrogate(codepoint);
        resultArr[resultPos + 1] = DecodeUtil.lowSurrogate(codepoint);
      }
  
      /**
       * Returns whether the byte is not a valid continuation of the form '10XXXXXX'.
       */
      private static boolean isNotTrailingByte(byte b) {
        return b > (byte) 0xBF;
      }
  
      /**
       * Returns the actual value of the trailing byte (removes the prefix '10') for composition.
       */
      private static int trailingByteValue(byte b) {
        return b & 0x3F;
      }
  
      private static char highSurrogate(int codePoint) {
        return (char) ((MIN_HIGH_SURROGATE - (MIN_SUPPLEMENTARY_CODE_POINT >>> 10))
            + (codePoint >>> 10));
      }
  
      private static char lowSurrogate(int codePoint) {
        return (char) (MIN_LOW_SURROGATE + (codePoint & 0x3ff));
      }
    }
  
    private Utf8() {}
  }
  